I've had this happen twice on Toyotas. I think it was from abuse. The customer was extremely rough on their machines. They would use them to settle pellets in huge boxes by lifting them up and shaking them violently by holding throttle wide open and jerking the tilt lever back and forth as fast as possible. I was able to just buy that aluminum end plate from Toyota and all was good. I think the pump is probably not quite strong enough and maybe the releif vavle doesn't react quick enough but once they were fixed they didn't keep doing it like the problem you have.